A coated type magnetic recording medium that has been widely used is prepared by providing a non-magnetic support with a magnetic recording layer containing magnetic particles of oxides such as .gamma.-Fe.sub.2 O.sub.3, Co-doped .gamma.-Fe.sub.2 O.sub.3, Fe.sub.3 O.sub.4, Co-doped Fe.sub.3 O.sub.4, a Bertholide compound of .gamma.-Fe.sub.2 O.sub.3 and Fe.sub.3 O.sub.4, or CrO.sub.2, and ferromagnetic alloy particles which are dispersed in an organic binder such as a copolymer of vinyl chloride-vinyl acetate, a copolymer of styrene-butadiene, an epoxy resin or a polyurethane resin, followed by drying the magnetic recording layer. Recently, attention has been directed to a magnetic recording medium of a thin metal film type comprising a magnetic recording layer of a thin ferromagnetic metal film prepared by a method wherein the film is formed in a vacuum chamber, such as by vacuum deposition, sputtering, or ion-plating, or by a metal plating method such as electro-plating or electroless plating, and has become of great commercial value with increasing demand for high density recording.
The conventional coated type magnetic recording medium uses mainly magnetic particles of metal oxides having low saturation magnetization. Therefore, when the magnetic recording layer is made thinner to realize high density recording, output of signals is decreased. On the other hand, in a magnetic recording medium of a thin metal film type, a ferromagnetic metal having higher saturation magnetization than magnetic oxides is used, and without using a non-magnetic substance such as a binder, the metal film can be fabricated to form an extremely thin magnetic recording layer. Accordingly, the electromagnetic properties thereof are excellent.
However, the magnetic recording medium of a thin metal film type has serious problems in that (1) friction resistance with parts such as a magnetic head or a guide pole is high upon recording, replaying, and erasing magnetic signals, and therefore wear resistance is not good, (2) the magnetic medium easily corrodes at high temperature and high humidity (e.g., at 40.degree. C. and 80 RH %) and (3) the magnetic recording layer is easily damaged by shock during handling.
It has been proposed to provide a protective layer on the magnetic recording medium of a thin metal film type to solve the above problems.
One such proposal is disclosed in Japanese Patent Application (OPI) No. 75001/75, and involves coating a lubricating agent on the surface of a thin metal film. In accordance with this method, the friction coefficient between a magnetic head or a guide pole and a thin metal film decreases, running properties of the tape become more stable and scratches are not easily formed. However, those effects abruptly decrease when a tape is used repeatedly.
It has also been known, as described in Japanese Patent Application (OPI) Nos. 39708/78 and 40505/78 that a protective layer such as a metal layer or a metal oxide layer can be formed on a thin metal film to lubricate it. In this case, the effect of the protective layer does not last long. When a tape of such a thin metal film type is repeatedly used, the friction coefficient abruptly increases, and the thin magnetic metal film tends to be easily damaged.
Another proposal, as disclosed in Japanese patent application (OPI) No. 155010/79 is that a film of a high molecular weight substance be overcoated on the thin metal film layer. In this method, when a high molecular weight substance such as a conventionally known copolymer of vinylidene chloride and acrylate is used, the thickness of the film is necessarily about 0.2 .mu.m, causing a spacing loss which unfavorably causes decreases output for high density recording.
It is often the case that an extremely smooth support for a thin magnetic metal film is used for high density recording. However in this case, running properties, particularly under highly humid conditions, and wear resistance are not satisfactory.